The present invention relates to a single optical disk unit for recording and/or reproducing data on or from two or more kinds of optical disks with different corresponding wave lengths.
An optical disk unit is an information recording and reproducing apparatus having characteristics as a medium such as noncontact, large capacity, random access, and low cost, and is widely used as a recording and reproducing apparatus of a digital audio signal or an external storage for a computer using these characteristics.
As a large capacity of computer data and recording and reproducing of digital moving picture information have been put to practical use recently, a high-density storage capacity of an optical disk unit is required. It may be cited as one of the means for realizing high density to make an optical spot focusing on the recording surface of an optical disk smaller and improve the resolution. The size of the optical spot is decided by the wave length of a laser beam source to be used and the numerical aperture NA of an objective lens, and when the wave length to be used is shortened, the diameter of the optical spot can be made smaller. In a DVD (digital video disk, ROM) standardized as a high density optical disk, it is decided to set the wave length to be used to 635 nm or 650 nm. In this case, the numerical aperture NA is n sin .theta., where n indicates a refraction factor on the emission side.
However, when an optical disk recorded in correspondence with the conventional wave length, 780 nm is reproduced using a laser beam with a short wave length of 635 nm or 650 nm, there is a problem imposed that a playback signal and control signal lowers due to differences in the reflection factor and absorption factor of the information recording surface. For example, in the standard of CD-R which is a write enable CD, the reflection factor is specified as 65% or more at a wave length between 775 and 820 nm. However, the reflection factor at a wave length beyond the specified range is low and may be, for example, about 5% in the neighborhood of 635 nm. On the other hand, the reproduction power is specified as 0.7 mW or less, so that it is difficult to obtain a sufficient playback signal and control signal when a laser beam with a short wave length is used.
According to the prior art described in Japanese Patent Application Laid-Open 8-55363, an optical system using a short wave length laser beam corresponding to a high density optical disk (hereinafter abbreviated to high density disk optical system) and a conventional optical system using a long wave length laser beam corresponding to a conventional CD or CD-R (hereinafter abbreviated to CD optical system) are loaded on an optical head, and an optical disk with a different corresponding wave length can be reproduced by switching the optical system depending on the disk to be reproduced. Furthermore, a common portion is provided in the high density disk optical system and the CD optical system using a beam synthesize/split means such as a polarized beam splitter, and the optical head is miniaturized.
In the aforementioned prior art, there are a constitution that the light flux synthesized by a beam synthesize/split means 23 is led to a beam convergent means 21 using a beam reflect means 22 such as a mirror as shown in FIG. 10a, and a constitution that there is no beam reflect means and the light flux synthesized by the beam synthesize/split means 23 is directly led to the beam convergent means 21 as shown in FIG. 10b.
In the constitution shown in FIG. 10a, the non-common portions of the high density disk optical system and the CD optical system are arranged in the same plane parallel with the disk surface, so that the optical head is increased in the length direction and width direction. In the constitution shown in FIG. 10b, one of the high density disk optical system and the CD optical system is arranged in the plane including the optical axis of the objective lens, so that the optical head is increased in the thickness direction. As mentioned above, it may be difficult to load the optical head by the prior art in an optical disk unit in which the size is limited in the length, width, and thickness directions, such as one to be built in a personal computer or an audio system mounted on a car.